1. Field of the Invention
The invention relates to a rolling bearing.
2. Description of Related Art
In a rolling bearing including an inner ring, an outer ring and a plurality of rolling elements rollably arranged between an inner ring raceway surface of the inner ring and an outer ring raceway surface of the outer ring, a plurality of dynamic pressure generating grooves may be formed in the raceway surface in order to reduce torque of the rolling bearing. Further, as described in Japanese Patent Application Publication No. 2009-108963 (JA 2009-108963 A), there is a rolling bearing in which multiple grooves having a finite length that corresponds to a width of each of the rolling elements in a direction orthogonal to a rolling direction of the rolling elements, are formed in a contact region of the raceway surface, at which the rolling elements contact the raceway surface. With this configuration, lubrication oil is retained in the grooves and therefore hardly flows out, so that dynamic pressure is generated.
The inner ring and the outer ring of the rolling bearing are subjected to precision machining in order to form their raceway surfaces into substantially true circles. However, the raceway surfaces have slight waviness, which cause vibrations during rotation of the bearing. The (amplitude of) vibration of the rolling bearing becomes larger when the number of the waves is equal to or nearly equal to an integer multiple of the number of the rolling elements. That is, the (amplitude of) vibration of the rolling bearing becomes larger when a relationship expressed by X=nZ or X=nZ±1 is established, where X is the number of waves, Z is the number of rolling elements, and n is a natural number (1, 2, 3 . . . ).
When multiple dynamic pressure generating grooves are formed in the raceway surface so as to be arranged at equal intervals in the circumferential direction, the dynamic pressure generating grooves cause the action similar to that of the waviness. Thus, when a relationship expressed by A=nZ or A=nZ±1 is established, where A is the number of waves, Z is the number of rolling elements, and n is a natural number (1, 2, 3 . . . ), the (amplitude of) vibration of the rolling bearing becomes larger, which may exert negative influence on reduction of torque of the rolling bearing and the durability of the rolling bearing.